POLLEN AND SPORES — LEOPOLD AND SCOTT 



307 



casionally by birds or other agents. The mechanisms of dispersal 

 have been important in the evolution of the number and type of pollen 

 grains produced by each plant species. Because wind is a random 

 agent in comparison with insects, whose travels about the plant 

 usually are motivated, production of enormous numbers of pollen 

 grains has definite survival value among wind-pollinated plants. In 

 addition to being produced in greater numbers per flower, pollen 



PINE 

 SPRUCE 

 JUNIPER 

 FIR 



GRASS 

 CATTAIL 



POLLEN PRODUCTION 



LOG GRAINS PER FLOWER 



K) 100 1.000 10,000 !00,0C 



SPEED OF FALL 



/ 



p 



MONOCOTS WHW J 



POLLINATED^ 



P 



CERTAIN DICOTS 



P 



u 



(DATA FROM DYAKOWSKA, 1937; KNOLL, 1952, ETC.) 



Figure 2. — The approximate numbers of pollen grains produced per flower, and the buoy- 

 ancy of single grains (measured by rate of fall in air) for some common plants. 



adapted for wind dispersal is usually lighter and less sticky than 

 that adapted for transport by insects. A quantitative comparison 

 of pollen production for some common wind- and insect-pollinated 

 plants, and specific gravity of grains as measured by rates of fall in 

 air, is shown in figure 2. Some insect-pollinated plants do produce 

 a large amount of pollen (e. g., willow), and some pollen adapted for 

 transport by insects is carried by air currents and deposited in en- 

 vironments favorable for its preservation. Nevertheless, the differ- 



